Thermal solvents in dry photothermographic systems
Heat processable photosensitive elements can be constructed so that after exposure, they can be processed in a substantially dry state by applying heat. It is known how to develop latent image in a photographic element not containing silver halide wherein organic silver salts are used as a source of silver for image formation and amplification. Such processes are described in U.S. Pat. Nos. 3,429,706 (Shepard et al.) and 3,442,682 (Fukawa et al.). Other dry processing thermographic systems are described in U.S. Pat. Nos. 3,152,904 (Sorenson et al.) and 3,457,075 (Morgan and Shely). A variety of compounds have been proposed as "carriers" or "thermal solvents" or "heat solvents" for such systems, whereby these additives serve as solvents for incorporated developing agents, or otherwise facilitate the resulting development or silver diffusion processes. Acid amides and carbamates have been proposed as such thermal solvents by Henn and Miller (U.S. Pat. No. 3,347,675) and by Yudelson (U.S. Pat. No. 3,438,776). Bojara and de Mauriac (U.S. Pat. No. 3,667,959) disclose the use of nonaqueous polar solvents containing thione, --SO.sub.2 -- and --CO-- groups as thermal solvents and carriers in such photographic elements. Similarly, La Rossa (U.S. Pat. No. 4,168,980) discloses the use of imidazoline-2-thiones as processing addenda in heat developable photographic materials.
Thermal solvents for use in substantially dry color photothermographic systems have been disclosed by Komamura et al. (U.S. Pat. No. 4,770,981), Komamura (U.S. Pat. No. 4,948,698), Aomo and Nakamaura (U.S. Pat. No. 4,952,479), and Ohbavashi et al. (U.S. Pat. No. 4,983,502). The terms "heat solvent" and "thermal solvent" in these disclosures refer to a non-hydrolyzable organic material which is a liquid at ambient temperature or a solid at an ambient temperature but melts together with other components at a temperature of heat treatment or below but higher than 40.degree. C. Such solvents may also be solids at temperatures above the thermal processing temperature. Their preferred examples include compounds which can act as a solvent for the developing agent and compounds having a high dielectric constant which accelerate physical development of silver salts. Alkyl and aryl amides are disclosed as "heat solvents" by Komamura et al. (U.S. Pat. No. 4,770,981), and a variety of benzamides have been disclosed as "heat solvents" by Ohbayashi et al. (U.S. Pat. No. 4,983,502). Polyglycols, derivatives of polyethylene oxides, beeswax, monostearin, high dielectric constant compounds having an --SO.sub.2 -- or --CO-- group such as acetamide, ethylcarbamate, urea, methylsulfonamide, polar substances described in U.S. Pat. No. 3,667,959, lactone of 4-hydroxybutanoic acid, methyl anisate, and related compounds are disclosed as thermal solvents in such systems. The role of thermal solvents in these systems is not clear, but it is believed that such thermal solvents promote the diffusion of reactants at the time of thermal development. Masukawa and Koshizuka disclose (U.S. Pat. No. 4,584,267) the use of similar components (such as methyl anisate) as "heat fusers" in thermally developable light-sensitive materials.